Pesticides can be formulated in the form of solid or liquid formulation. Solid formulation include dry flowables (DF), granules, wettable powders, soluble granules, water dispersible granules, effervescent granules and so on. Liquid formulation include soluble concentrates, suspension concentrates, emulsifiable concentrates, micro-emulsion, oil based suspension concentrate or suspoemulsion and so on. Type or nature of the active ingredient is very important in deciding the type of liquid formulation. Water soluble active ingredients are formulated as soluble concentrates (SL) and water insoluble active ingredients are formulated in the form of suspension concentrates (SC). For a combination of water soluble active ingredient and a water insoluble active ingredient the preferred formulation type is suspoemulsion or microemulsion.
Water soluble active ingredients include the active ingredients which are capable of forming a salt. If the active ingredient has an acid group, for example a carboxylic acid group (COOH), sulfonic acid group (—SO3H) or phenolic group (—OH), they can form a salt with counter ions of alkali metals like calcium, magnesium or with counter ions of ammonia, alkyl amine, dialkyl amine or trialkyl amine. If the active ingredient has a basic group like an amine or a substituted amine it can form a salt with counter ion like hydrogen, a carboxylate, a sulfonate or a phosphate compound.
The liquid formulations in the form of soluble concentrate are clear solution dissolved in it is the water soluble active ingredient in the form of salt and other soluble surfactants and/or organic solvents. This formulation may contain a co-solvent to maintain homogeneity of the formulation. These formulations often suffer from a drawback as they tend to solidify as the storage temperature decreases. It is understood that when the active ingredients and/or adjuvants are solids at room temperature, at lower temperatures, their aqueous formulations have the tendency for crystallisation of the solids present in it. Such formulations when stored under cooler climatic conditions require further warming up of the formulation to make it homogenous to enable further use. These procedures are often unsafe and may affect the bioefficacy of the product. There can be uneven phase resulting in uneven distribution of the active ingredient and the surfactant resulting into uneven, inadequate and often insufficient application of the active ingredient resulting into ineffectiveness of formulation.
This problem is more aggravated when a liquid aqueous formulation contains one or more salt forming pesticides. If there is a solid separation, the formulation becomes inconsistent, non-homogenous and difficult to handle. It will also result into uneven distribution of active ingredients resulting in poor control of weeds.
Examples of such herbicides include nitrophenyl ether herbicides such as acifluorfen and benzothiadiazine herbicide such as bentazone.
Acifluorfen and its preferred derivatives are from the class of nitrophenyl ether herbicides (diphenyl ethers) and act as Protoporphyrinogen oxidase inhibitor. It is a selective contact herbicide, absorbed by the foliage and roots and is used as a post-emergence herbicide for the control of annual broad-leaved weeds (Abutilon, Amaranthus, Datura, Euphorbia, Polygonum, Ipomoea, Xanthium spp.), on grasses in soya beans, peanuts and rice. Usually it is applied in the form of sodium salt and is formulated as soluble concentrate which is sold under the brand ‘Blazer’.
Bentazone is a selective contact herbicide, absorbed mainly by the foliage, but also by the roots. It is a photosynthetic electron transport inhibitor at the photosystem II receptor site. Bentazone is used in controlling Anthemis, Chamomilla and Matricaria spp., Chrysanthemum segetum, Galium aparine, Lapsana communis and Stellaria media in winter and spring cereals. Other crops include peanuts, maize, peas, Phaseolus beans, rice (Cyperus difformis, C. esculentus, C. serotinus, Monochoria vaginalis, Sagittaria pygmaea, S. sagittifolia, Alisma and Commelina spp., Scirpus maritimus and S. mucronatus) and soya beans (Abutilon theophrasti, Capsella bursa-pastoris, Cyperus esculentus, Datura stramonium, Helianthus spp., Polygonum spp., Portulaca spp., Sida spinosa, Ambrosia spp., Sinapis arvensis and Xanthium spp.).
Combinations of particular interest are that of acifluorfen and bentazone as their activity is complementary—an effective control of most of the weeds can be achieved. Usually acifluorfen is applied in the form of sodium salt and is formulated as soluble concentrate which has a freezing point of 5° C. Since the freezing point is on a higher side, precaution has to be taken while making the formulation as it leads to crystallisation of solid (or freezing of solution) between 10-0° C. This problem aggravates when acifluorfen is required to be used in combination with other active ingredients. For example, when acifluorfen is combined with another salt solution of bentazone a significant problem normally occurs. The problem is the sedimentation or tendency for crystallisation of the active ingredients at lower temperatures. Antifreeze agents/crystal inhibitors are generally used to solve the problem of solidification at low temperature. Generally used antifreeze agents include glycols like propylene glycol, and crystal inhibitors include Rhodafac rs-610, K: Atlox AL-3382 and Agsole Ex.
‘Galaxy’ and ‘Storm’ are the two commercially available compositions containing acifluorfen sodium and bentazone sodium. However both the formulations fail the freeze thaw test meaning that both the formulations are suffering from the problem of solidification at low temperatures.
It has been observed by the inventors of the present invention that while using known antifreeze agents such as glycol and glycol based solvents alone could not solve the problem of low temperature stability. It has also been noted by the present inventors that use of known thickening agents also leads to solidification problems. Additionally, the known thickening agents led to high viscous aqueous concentrates which were creating difficulties in pouring and/or pumping the formulation in the field especially at low temperatures. These formulations when applied in the field may lead to uneven distribution of the active ingredients resulting in poor bioefficacy.
It would therefore be highly desirable to have aqueous based formulations which are stable especially at low temperatures.
Inventors of the present invention have surprisingly found out that a stable liquid composition of pesticides in their salt form can be prepared by way of adding a crystal habit modifying system.